Fluoro-olefins, such as VF3, are known and are used as monomers or comonomers in the manufacture of fluorocarbon polymers exhibiting noteworthy characteristics, in particular an excellent chemical strength and a good thermal resistance.
Trifluoroethylene is a gas under standard pressure and temperature conditions. The main risks related to the use of this product relate to its flammability, its propensity to self-polymerization when it is not stabilized, its explosiveness due to its chemical instability and its supposed sensitivity to peroxidation, by analogy with other halogenated olefins. Trifluoroethylene exhibits the distinguishing feature of being extremely flammable, with a lower explosive limit (LEL) of approximately 10% and an upper explosive limit (UEL) of approximately 30%. However, the major danger is associated with the propensity of VF3 to violently and explosively decompose under certain pressure conditions in the presence of an energy source, even in the absence of oxygen. Tests carried out by the applicant company in order to determine the limiting stability pressure (Pst) of VF3 (maximum pressure for which there is no ignition) have made it possible to determine the Pst of VF3 at 4 bar. In the event of explosion, under the conditions of the test, the Pex/Pi excess pressure ratio is approximately 10. For its part, the minimum ignition energy is unknown. This is why it is essential to avoid any point heat source, such as that resulting from the uncontrolled exothermic polymerization (self-polymerization). Finally, as VF3 is a halogenated ethylenic compound, it is included among the peroxidizable compounds. The risk of peroxidation and also the risk of self-polymerization increase in the presence of a liquid phase. There is a risk of explosion as a result of a peroxidation or of an initiation of polymerization during the storage of this type of molecule.
In view of the main risks above, the synthesis and the storage of VF3 present specific problems and impose strict safety rules throughout these processes.
Several routes for the synthesis of VF3 are described in the literature.
A first route, for example described in the document EP 485 246, consists of the hydrogenolysis of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) carried out in the gas phase in the presence of a mixed catalyst based on copper or on silver and on at least one metal from the platinum group (ruthenium, rhodium, palladium, osmium, iridium or platinum). While the starting materials are readily available, the main product from this technique is CTFE, VF3 being only a byproduct. The lifetime of these catalysts is relatively short and it is difficult to obtain good selectivity for VF3 if the conversion of the CFC-113 is increased. The VF3 yields are thus low.
A second route for the preparation of trifluoroethylene is based on the catalytic dehydrofluorination of tetrafluoroethane HFC-134a. The document FR 2 729 136 thus describes a process which employs aluminum fluoride as catalyst. The trifluoroethylene is obtained by catalytic dehydrofluorination in the presence of BF3. The degree of conversion of the HFC-134a is 18.5%. The costs of the catalyst for this technique are low and there is no need to inject hydrogen, which simplifies the collection of products. However, the operating conditions are difficult and the degree of conversion of the HFC-134a is low, just like the trifluoroethylene yield.
A third route for the preparation of trifluoroethylene is represented by the debromination/dechlorination reaction of 1,1,2-trifluoro-2-chloro-1-bromoethane, described, for example, in document JP57026629. The reaction takes place in the presence of water and of a dehalogenating agent (for example Zn). The reaction conditions are mild, but the starting materials are difficult to find and the reaction produces a great deal of effluents.
A fourth route for the preparation of trifluoroethylene, for example described in the document U.S. Pat. No. 5,892,135, resorts to saturated halogenated hydrocarbons of CF3CClFX type (X being H, Cl or F), such as 124, 114a and 115, in the presence of a catalyst composed of one or more metal elements, such as Ru, Cu, Ni, Cr, or of their metal oxides or halides. The degree of conversion of the CF3CClFX can reach 91%, while the selectivity of trifluoroethylene can reach 83%, the remainder being 1132a, HFC-134a and 1122. This technique for the manufacture of trifluoroethylene exhibits a relatively high yield; however, the reaction temperature is high (between 325-425° C.) and the catalyst easily loses its activity. It also exhibits difficulties in collecting, separating and purifying the reaction products.
A fifth known route for the preparation of trifluoroethylene uses, as starting materials, chlorotrifluoroethylene (CTFE) and hydrogen in the presence of a catalyst having, as active components, metals from Group VIII and a support composed of porous materials, such as active charcoal, alumina, titanium oxide, magnesium oxide, magnesium fluoride and aluminum fluoride.
The catalytic hydrogenolysis of CTFE is generally carried out in the gas phase. For example, the document U.S. Pat. No. 3,564,064 describes a catalyst based on Pd or Pt on a support of active charcoal or alumina. The reaction temperature is between 200 and 320° C., with a contact time of 0.1 to 4 seconds. The gases resulting from the reaction are washed in water and a base and then dried with anhydrous calcium sulfate. The products are recovered by condensation in a trap cooled by a methanol/dry ice mixture and are then purified by fractional distillation of the mixture recovered in the trap cooled by acetone/dry ice. The degree of conversion of the CTFE is more than 60%, with a selectivity for VF3 of more than 80%.
However, the catalytic hydrogenolysis of CTFE can be carried out in the liquid phase in the presence of a hydrochloric acid acceptor and of a metal from Group VIII, as described in the document CN 1 080 277. The acid receptor is an alcohol, an amine, an ester or an ether; the degree of conversion of the CTFE reaches 100%, with a selectivity for VF3 of 80-90% and a yield of 60-90%. In addition to the trifluoroethylene, difluoroethylene, 1,1,2-trifluoroethane and 1,1-difluoroethane are obtained as byproducts.
On comparing the technique for the production of trifluoroethylene from CTFE with that which starts from CFC-113, there is a decrease in the reaction products, with a relatively large increase in the trifluoroethylene yield; however, problems of lifetime of the catalyst and the difficulties in collecting, separating and purifying mentioned for the other routes also exist with this technique. In point of fact, these stages have to be compatible with the reaction being carried out on an industrial scale.
There thus exists a real need to develop an alternative process for the preparation of trifluoroethylene from CTFE which makes it possible to overcome the above-mentioned disadvantages so as to obtain VF3 in an economical manner under conditions which limit as far as possible the risks of explosiveness of this molecule.
It has now been found that, for the hydrogenolysis of CTFE, the use of a catalyst based on a metal from Group VIII and more particularly based on Pd deposited on a support and also a particular sequence of separation and purification stages makes it possible, at atmospheric pressure and at relatively low temperatures, to obtain excellent degrees of conversion of the CTFE and of selectivity for VF3.